Romans 8:20 (KJV)

For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.

The Spirit itself beareth witness with our spirit, that we are the children of God:

And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.

For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us.

For the earnest expectation of the creature waiteth for the manifestation of the sons of God.

Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.

For we know that the whole creation groaneth and travaileth in pain together until now.

And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.

For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for?

But if we hope for that we see not, then do we with patience wait for it.